Carbon isotopes and iodine concentrations in a
Mississippi River delta core recording land use, sediment
transport, and dam building in the river’s drainage basin
Peter H. Santschi, Sarah D. Oktay, Luis Cifuentes
To cite this version:
Peter H. Santschi, Sarah D. Oktay, Luis Cifuentes. Carbon isotopes and iodine concentrations
in a Mississippi River delta core recording land use, sediment transport, and dam building in
the river’s drainage basin. Marine Environmental Research, Elsevier, 2007, 63 (3), pp.278. .
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Accepted Manuscript
Carbon isotopes and iodine concentrations in a Mississippi River delta core re
cording land use, sediment transport, and dam building in the river’s drainage
basin
Peter H. Santschi, Sarah D. Oktay, Luis Cifuentes
PII:
DOI:
Reference:
10.1016/j.marenvres.2006.11.002
MERE 3078
To appear in:
Marine Environmental Research
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Revised Date:
Accepted Date:
3 January 2006
13 November 2006
13 November 2006
S0141-1136(06)00207-8
Please cite this article as: Santschi, P.H., Oktay, S.D., Cifuentes, L., Carbon isotopes and iodine concentrations in
a Mississippi River delta core recording land use, sediment transport, and dam building in the river’s drainage basin,
Marine Environmental Research (2006), doi: 10.1016/j.marenvres.2006.11.002
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ACCEPTED MANUSCRIPT
Carbon isotopes and iodine concentrations in a Mississippi River delta core recording land
use, sediment transport, and dam building in the river's drainage basin
Peter H. Santschi1, Sarah D. Oktay1,2, and Luis Cifuentes3
1) Laboratory for Oceanographic and Environmental Research (LOER), Dept. of Marine
Sciences and Oceanography, Texas A&M University at Galveston, 5007 Ave U, Galveston, TX
77551 ([email protected]).
2) University of Massachusetts - Boston, Nantucket Field Station, Grace Grossman
Environmental Center, 180 Polpis Rd, Nantucket, MA 02554 ([email protected]).
3) Department of Oceanography, Texas A&M University, College Station, TX 77843
([email protected]).
Marine Environmental Research
submitted: January 2006
revised: October 2006
Keywords
Mississippi River delta; organic carbon; sediment transport; land use, radiocarbon; C-13; iodine;
salt marsh
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Abstract
Sedimentary material from coastal and nearshore areas in the Mississippi Delta region are
comprised of different organic carbon sources with diverse ages that require isotopic and
elemental records for resolving the various sources of plant residues. Carbon isotopic (13C, 14C)
values were used to differentiate contributions from plants using the C3, C4, and/or CAM
(crassulacean acid metabolism) carbon fixation pathways., and iodine concentrations indicated
that wetland plant residues are a significant source of organic carbon in a sediment core from the
Mississippi River Delta region collected at a 60 m water depth. This sediment core had been
extensively described in Oktay et al. (2000), and significantly, includes unique features that had
not previously been seen in the marine environment. These special features include a plutonium
isotopic close-in fallout record that indicates a purely terrestrial source for these sediment
particles and the elements associated with it, and a distinct iodine isotopic peak (as well as peaks
for plutonium and cesium isotopes) that indicate little bioturbation in this core. Our carbon
isotopic and iodine data can thus be compared to published records of changes in drainage basin
land use, river hydrology, and hydrodynamic sorting of suspended particles to elucidate if these
changes are reflected in nearshore sediments. This comparison suggests a significant contribution
for organic carbon (OC) from C4 plants to these sediments during the 1950’s to early 1960’s.
Relative older carbon isotopes, and episodically high iodine concentrations (up to 34 ppm) were
observed during this time period that 1) indicate sediment deposition that is coincident with the
times of major hydrological changes induced from dam and levee building in both the upper and
lower reaches of the Mississippi River drainage basin, and 2) suggest episodic organic carbon
deposition from wetland plant residues.
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Introduction
The Mississippi River watershed is the largest of the contiguous United States, draining
approximately 40% of its surface area. The Mississippi River is also by far the largest source of
sediment and organic matter to the Gulf of Mexico. In its delta region, the potential exists for
reconstructing the history of organic matter sources eroded from a large part of the U.S. drainage
basins through analysis of particulate organic carbon, POC, in radioisotopically-dated sediment
cores. There have been relatively few attempts to interpret organic carbon isotopic records in
terms of its sources and in a historical context (e.g., Eadie et al., 1994). Since suspended
sediment loads change in response to variations in hydrological conditions along the river, and
suspended sediments reaching the delta and shelf regions of the Gulf of Mexico are sorted
hydrodynamically (Bianchi et al., 2002), comparing organic carbon source datum in Mississippi
Delta area sediment cores with the drainage basin historical record will have to take into account
both changes in hydrological conditions as well as hydrodynamic regimes.
Of particular interest are changes in hydrological conditions such as the construction of dams and
levees that were installed to reduce flooding or accommodate increases in traffic on the river. As
part of the New Deal, Roosevelt backed the construction of locks and dams in order to improve
trade and transportation and provide work for Americans struggling to survive after the
Depression. For instance, the Fort Peck dam was constructed on the Upper Missouri in Montana
between 1933 and 1940, and at the time it was built was the 5th largest dam in the world and is
still one of the world’s largest earth-filled dams. In 1953, flood control dam construction began
again on the Missouri River and all dam construction was completed by 1967. The Missouri
River which joins the upper Mississippi near St Louis has always been the primary contributor of
sedimentary material to the Mississippi, but this total contribution was reduced by 70% after the
construction of these dams (Keown et al., 1986). The average annual sediment load has
decreased three-fold as measured at a station in St, Louis Missouri, with a reduction from the
1953 value of 3.19x108 tons per year to 1.1x108 tons per year in 1967. Dissolved silicate loads
decreased as well due to removal behind newly constructed dams (Carey et al., 1999). This large
decrease in sediments from damming the western tributaries (Missouri and Arkansas rivers) was
counterbalanced by a significant increase in sediment loads from the Ohio River resulting from
deforestation and rowcrop farming (Keown, 1986).
As a consequence of the reduced loads from the Missouri River, the sediment load determined at
Tarbert Landing along the main stem of the Mississippi River flowing into the Gulf of Mexico
decreased from 3.0x108 tons/yr from 1950-1962 to 1.6x108 tons/yr for the period 1970-1978.
Concurrent to the changes in flood control dams for the Mississippi River in the upper drainage
basin, Old River Control Structures were constructed to regulate the flow to the Atchafalaya
River in order to partition the flow between the main stem and the Atchafalaya River and prevent
rapid silting of navigation channels in the lower Mississippi River. They were completed and
became operational in 1963.
According to Keown et al. (1986), approximately 75% of the total sediment discharged from the
lower Mississippi River was derived from the Missouri River before the 1950’s. Recent work has
shown that bulk particle organic carbon (POC) derived from the dominant sources of grasslands
in the Missouri River watershed has a lignin signature that is typical of non-woody angiosperm
sources (Onstad et al., 2000). Erosion of soils of the Mississippi River drainage basin with
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extensive grasslands containing isotopically heavier C4 plants is a main contributor of POC in
the Mississippi River (Goni et al., 1997). Downcore variations of the δ13C signature of POC from
a site located in the Mississippi River Delta region were previously interpreted as changes
between marine productivity and terrestrial organic matter inputs (Eadie et al., 1994). However,
these data can be reinterpreted using the data of Goni et al. (1997) by considering the
contribution of terrestrial POC from C4 plants that has increased during the past century. Land
use changed in the Mississippi River Basin from the predominantly grassland (C4)-forest settings
of the early 19th century to the current intensive use of over one third of the land in the basin for
agricultural purposes (Keown et al., 1986).
A mechanism has been proposed by Bianchi et al. (2002) that can explain the observations of
POC deposition in riverine and nearshore regions of woody C3 plant derived carbon, and of C4
plant-derived POC further offshore in shelf and slope sediments. These authors suggested
hydrodynamic sorting of POC in Mississippi River Delta regions and the shelf waters, primarily
as a result of frequent resuspension of sediments in the river bed caused by changes in river
discharge. This would cause fine-grained sediments to be winnowed out in the water column and
transported downstream, while the larger sediments with woody material associated with it and
with higher relative densities would selectively settle out in the lower river and inner shelf
sediments.
Changes in erosion river beds near New Orleans have also occurred in the 1960s in response to
changes in subsidence and river hydrology (Galler et al., 2003). This has exposed peat deposits
with radiocarbon ages of 2000-4000 years, and δ13C values of -24.6 to -25 ‰. Keppler et al.
(2004) found that peatlands sequester large amounts of iodine with average concentrations of 536 mg/kg, therefore, one could recognize such layers in a sedimentary record by their unusually
high iodine concentrations.
There are other organic matter sources with high iodine concentrations, however, such as salt
marsh vegetation, which typically have heavy δ13C signatures due to C4 photosynthetic
pathways (Hoefs, 1987; Fuge and Johnson, 1986). Thus, the combined use of carbon isotopes
and iodine concentrations become promising tools to differentiate peat from salt marsh organic
matter sources.
Here, we will compare organic carbon isotopic and iodine records that are imprinted into a
previously described Mississippi River Delta sediment core (Oktay et al., 2000). This nearshore
(60 m water depth) sediment core from the Mississippi River delta region is unique in that it has
preserved the terrestrial close-in plutonium bomb fallout record that has not been documented in
any previously published marine record. Early US bomb tests at the Nevada test site conducted
in the early 1950’s were low-yield and near-ground, moving large radioactive dust clouds from
240
239
Nevada across the continental USA, which received fallout with low Pu/ Pu ratios of 0.05 and
lower. While marine sediment Pu isotopic ratios below the sediment surface have only
documented global bomb fallout 240Pu/239Pu ratios of 0.18, the sedimentary record of Pu isotopes
by Oktay et al. (2000) is, to our knowledge, the only marine record of the terrestrial close-in
fallout from the Nevada test site. Thus, the low Pu isotopic ratio deeper in this sediment core is
strong evidence that Pu and associated elements in this sediment core from this Mississippi River
delta site are mainly from terrestrial sources, with minimal marine input. Furthermore, the iodine
isotopic record from bomb fallout contains a sharp 1963 fallout peak that is preserved in this
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sediment core material, and thus clearly indicates a record with little alteration by benthic
bioturbation or iodine diagenesis.
Most importantly, this sediment core allowed us to compare the sedimentary organic carbon and
iodine signals with recorded changes in drainage basin land use, river hydrology, and
hydrodynamic sorting of suspended particles. Since coastal areas are subject to organic carbon
from different sources with different ages, the interpretation of any carbon isotopic record is
often a challenging task, and requires additional isotopic and elemental records (e.g., Santschi et
al., 2001a). The fallout isotopic records (e.g., 239Pu/240Pu, 137C, 129I/127I, and 210Pbxs) that are
available from the work of Oktay et al. (2000) thus provide a unique opportunity to investigate
the historical record of terrestrial organic matter inputs into the Mississippi River delta region
through the additional analyses of organic carbon isotopes (13C, 14C).
Methods and experimental approaches
Sampling
Two sediment cores from the Mississippi River Bight were taken during the 1993 Texas A&M
University research cruise 93P13 (aboard the R/V Pelican), approximately 24 km due west of the
mouth of the South West Pass of the Mississippi River (Figure 1). This location was chosen in
order to procure sediment cores with predictably high sedimentation rates based on their location
within the plume of particulate material that exits the Mississippi River as can be clearly seen in
Figure 1. The core location in the Mississippi River Delta region are: core 93P13 #2 at 28°55.5’
N, 89° 40.6’ W in about 60 m water depth in the vicinity of Southwest Pass, of which the
radiochemical results are described in Oktay et al. (2000), and carbon isotopic results described
here), and parallel core 93P13 #1 from the same general site, of which the persistent inorganic
and organic pollutants, as well as accompanying radiochemical results are described in Santschi
et al. (2001).
Analytical techniques
Isotopic analyses of δ13C and ∆14C were conducted on dry sediment core material. Five 500-600
1
mg sediment aliquots were analyzed for ∆ 4C by the Accelerator Mass Spectrometry (AMS)
facility at the Lawrence Livermore National Laboratory (LLNL), with relative standard
deviations of ±5-10‰ (corresponding to errors in radiocarbon ages of 50-100 years). δ13C was
measured on 100 mg aliquots of several depths throughout the core via conventional mass
spectrometry, according to Cifuentes et al. (1988) and Guo and Santschi (1997). The precision
and accuracy of the d13C analysis was better than 0.1 ‰, as determined from replicate analyses
of standards and samples (Guo et al., 1996); actual 1 standard deviation values are given in Table
1. Organic and inorganic carbon concentrations were obtained using a Perkin Elmer CHN/S
analyzer on 0.5 g sediment aliquots. The relative standard deviation of the particulate organic
carbon analysis was ± 1% (relative) as determined from replicate analyses of standards and
samples (Guo and Santschi, 1997), which would result in a standard deviation of 0.01 % at 1 %
OC. Stable iodine (I127) concentrations, previously published in Oktay et al. (2000), were
measured via ICP-MS using either a Sciex/Perkin Elmer ELAN 500 or a Hewlett Packard model
4500 instrument, after extraction of iodine from the sediments using an alkali leach and fusion
method (Nishizumi et al., 1983). The sediment samples were placed in alumina oxide crucibles
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with a saturated NaOH solution and dried overnight. Next, concentrated Na2O2 was added and
the mixture is heated in a muffle furnace, first at 300°C for 30 minutes and then at 600°C for 2
hours. De-ionized water, 1M NaHSO3 and 50% H2SO4 were added to help dissolve the majority
of the material. Finally, the sample was centrifuged and the supernatant decanted for iodine
processing (Oktay et al., 2000).
Analytical results are shown in Table 1. As δ13C data show only gradual changes, this allowed
interpolations for missing data points that were used to compare with •14C values.
Results and discussion
The 210Pbxs and 239,240Pu and/or 240Pu/239Pu geochronology of this sediment core makes it
possible to determine average sedimentation rates for certain depth intervals, and to assign dates
at three specific depths throughout the core: 1993 (surface), 1963 (at 21 cm; denoting the bomb
fallout peak) and 1953 (at 59 cm; beginning of significant fallout from atmospheric testing of
atomic bombs). Corresponding dates between these event horizons were assigned via linear
interpolation using the calculated sedimentation rate for each portion. 210Pbxs dating indicates a
break in sedimentation rates in the early 1960’s, as the sedimentation rate decreased from 2
cm/yr before that time to 0.7 cm/yr in recent years (Oktay et al., 2000), in accordance with a
decrease in sediment supply at the same time.
Radiocarbon and δ13C data (Figure 2) from organic carbon (OC) in this delta sediment core that
was previously dated using plutonium isotopic ratios and 210Pbxs (Oktay et al., 2000) shows not
only a minimum (Figure 2a) in 14C and maximum (Figure 2b) in δ13C, but also a maximum in
total iodine in layers dated as 1957 (Figure 3). Iodine in suspended and sinking sediments is
mostly from geochemically modified erosional inputs in response to higher river flow rates
(Oktay et al., 2001). 14C data from shelf environments receiving multiple sources of organic
carbon are more difficult to interpret than those from delta regions of large rivers such as the
Mississippi River. For example, a changing 14C signal from bulk POC can be a result of a
variable mixing ratio of old riverine and/or shelf organic carbon into a marine organic carbon
background source (e.g., Palos Verdes shelf; Santschi et al., 2001a).
While δ13C peaks in about 1957, and 14C shows a minimum at about the same time, 240Pu/239Pu
ratios are steadily decreasing in sediments older than 1963, and 137Cs required decay correction
to the time of deposition because of transport and drainage basin effects (Oktay et al., 2000).
Most important for lack of iodine mobility and lack of bioturbation, 129I/127I ratios exactly track
the Pu isotopic ratios (Figure 5 of Oktay et al., 2000), despite multiple sources. This indicates
that the 129I, 239Pu and 240Pu and other bomb fallout nuclides were from Mississippi River
drainage basin soils, deposited from close-in tropospheric fallout originating from the Nevada
test sites in the 1950’s (Oktay et al., 2000). Therefore sediments and POC in this section of the
core are clearly of terrestrial origin.
Thus, the observations of changing sedimentation rate, coupled to the change in carbon isotopic
signatures of sedimentary organic carbon can be interpreted relative to decreases in sediment
supply to the delta caused by anthropogenic modification of the river flow regime, as well as
changes in organic carbon sources.
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Interestingly, core 93P13#1, taken from a nearby site (Santschi et al., 2001b), showed similar
sedimentation rates over the past 50-100 years based on 210Pbxs and 239,240Pu peaks, i.e., 0.7
cm/yr, but did not show decreasing 240Pu/239Pu ratios below the peak. This suggests that the
depositional environment near the site our core (93P13 #2) was taken was uneven, possibly with
bottom flows of river sediment fingering out from the delta region. Inventories of particlereactive 239,240Pu in both cores from this site were enhanced over those expected from
atmospheric deposition, which is to be expected for a river delta region. While Pu isotope
inventories in sediments off the Mississippi River Delta are elevated and decrease with
increasing water depth (Yeager et al., 2004), this elevated Pu inventory in our core, together with
the unique signature of lower 240Pu/239Pu ratios in subsurface sediments, also agrees with our
contention of significant inputs of terrestrial material to the area our sediment core was taken.
Iodine concentrations in our core peak near a depth of 40 cm (dated to around 1957), and
periodically below, with significant offsets of these few data points from the trend line of iodine
vs. OC concentration (Figure 4). While the trend line shows molar I/C ratios of about 5x10-5, the
few high points with higher iodine concentrations have molar I/C ratios of about 20x10-5. No I/C
data is available from Mississippi River wetlands. However, both ratios appear somewhat higher
than reported I/C ratios in peat bogs from Southern Chile (e.g., 1x10-5; Keppler et al., 2004).
This suggests episodic deposition of organic material from a high iodine content, relatively low
carbon (e.g. in comparison to algae) source such as peat or marsh plant residues. Organic carbon
from such a source, while old, indicates C4 or CAM (crassulacean acid metabolism) origin, as
δ13C values for this section of the core are relatively heavy (-21.2 ‰), rather than -24 to -30 ‰
for the isotopically lighter C3 plants. Furthermore, in the 1956-1993 data set, 14C-age and
(interpolated) δ13C linearly and significantly correlate (Figure 5, Table 1), suggesting a mixing
behavior of at least two distinct carbon sources. While a correlation between 14C - ages and δ13C
values had been described previously by Gordon and Goni (2003) for nearshore sediments near
the Atchafalaya River, albeit with significantly more depleted δ13C values ranging from -21 to 27 ‰, characteristic for a large part of the shelf, their range of δ13C values is much larger (i.e., 21 to -27‰), and their linear correlation (i.e., slope and intercept) is significantly different.
Nonetheless, the points in our correlation still fall within the range of values Gordon and Goni
(2003) observed.
It might be possible that peat residues dated by 14C to be several thousand years old, exhibiting
isotopically depleted δ13C signatures (~ -25 ‰) that had been observed in eroding river beds near
New Orleans (Galler et al., 2003) could be the source material. However, because δ13C values in
our core peak at -21.2 ‰ and not -25 ‰, this possibility can be ruled out. Since δ13C values of 21.2 ‰ are coincident with higher iodine values, this suggests that our 13C record indicates
episodic deposition of more 13C- and iodine-enriched wetland deposits rather than peat residues
from old river beds deposited during the 1950’s. Wetland plants and their residues are known to
contain more isotopically heavy carbon (Hoefs, 1987) that are also enriched in iodine (Fuge and
Johnson, 1986). Thus, our sedimentary record suggests that eroded organic carbon from wetlands
has episodically been eroded and deposited into these delta sediments during those particular
times.
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While Goni et al. (1998) considered the possibility of marsh-derived organic matter as a source
of the terrigenous organic matter they observed in Gulf of Mexico sediments, they dismissed this
possibility for a number of reasons. They argued that the subsidence of the deltaic plain and
surrounding lands should make them a sink and not a source of sediments, and they expected that
marsh sediments should be younger in 14C-age than the old sediments they observed on the shelf
and slope. However, they only considered current conditions, and obviously, hydrological
conditions during the 1950’s and early 1960’s were different. Also, while our data suggests that
the OC originates from older wetland deposits that must have periodically been deposited in the
delta sediments at 60 m water depth, these layers with highest iodine concentrations appear to
account for only a minor fraction of the sedimentary deposits.
The significant on-shore – off-shore gradient in δ13C values on the Louisiana – Mississippi shelf,
with δ13C values of -25‰ in Mississippi and Atchafalaya Rivers and near-shore sediments, and 22 to -20 ‰ in outer shelf and slope surface sediments, has been explained by hydrodynamic
sorting of POC, with C3 plant material being heavier and depositing closer to shore, and C4 plant
residues associated with finer-grained particles that are being transported further off-shore
(Bianchi et al., 2002). Obviously, this general pattern does not differentiate contributions from
different sources of C4 plant residues, e.g., from grass or marsh lands. Only in combination with
iodine signatures, is it possible to distinguish deposits of iodine-enriched wetland-derived POC,
which have only small isotopic deviations in δ13C values from those of C4 plants.
Conclusions
The carbon isotope and iodine data from a Mississippi River delta core correlates not only with
changes in land use and decreases in sediment and OC deposition since the early 1960s, but also,
shown here for the first time, with episodic organic carbon deposition from older wetland plant
residues (which are known to contain high iodine concentrations). This was especially
pronounced for sediments deposited in the late 1950’s, when sediment deposition rates at this
site were higher. The δ13C maximum at the same depth also indicates a more significant
contribution for OC from C4 plants during the 1950’s to early 1960’s, coincident with the times
of major hydrological changes both in the upper and lower reaches of the Mississippi River
drainage basin. Most recently, hurricanes Rita and Katrina allowed a study of their effects on
wetlands in Lousiana (Turner et al., 2006), which concluded that hurricanes nourish rather than
erode wetlands. Recent sediments in our core contain younger (1560 years) and isotopically
lighter OC (-21.96 ‰), suggesting a lesser influence of wetland carbon deposits, in accordance
with the increased channelization of the Mississippi and Atchafalaya Rivers in recent times.
These findings, even though only documented in one sediment core, are significant as they allow
extrapolation to other large rivers that have been dammed and channelized in recent times.
However, more work needs to be carried out on inputs from wetlands and the use of 239Pu/240Pu,
δ13C and I/C ratios in sediment cores as tracers for inputs from different terrestrial environments.
Acknowledgements
The ∆14C results were analyzed by the Accelerator Mass Spectrometry (AMS) facility at the
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Lawrence Livermore National Laboratory (LLNL). This work was supported, in part, by the
Coordinating Board of Texas (Advanced Research Program, Grant number 010298-013) and the
Texas Institute of Oceanography.
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multiple radioisotopic tracers (137Cs, 239,240Pu, 210Pb, 234Th, 238U and 14C). Mar. Chem., 73(2),
125-152.
Santschi, P.H., Presley, B.J., Wade, T.L., Garcia-Romero, B., & Baskaran, M. (2001b).
Historical contamination of PAHs, PCBs, DDTs, and heavy metals in Mississippi River Delta,
Galveston Bay and Tampa Bay sediment cores. Mar. Environ. Res., 52(1), 51-79.
Turner, R.E., Baustian, J.J., Swenson, E.M., and Spicer, J.S. 2006. Wetland sedimentation from
hurricanes Katrina and Rita. Science, 314, 449-452.
Yeager, K.M., Santschi, P.H., & Rowe. G.T. (2004). Sediment accumulation and radionuclide
inventories (239,240Pu, 210Pb and 234Th) in the northern Gulf of Mexico, as influenced by
organic matter and macrofaunal density. Mar. Chem., 91, 1-14.
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Table 1. Carbon isotopic results from core 93P13 #2, along with % OC and iodine data from
Oktay et al. (2000). -: not determined; *: interpolated.
Depth
(cm)
0.5
(±0.5)
1.5
(±0.5)
2.5
(±0.5)
3.5
(±0.5)
4.5
(±0.5)
5.5
(±0.5)
6.5
(±0.5)
7.5
(±0.5)
8.5
(±0.5)
9.5
(±0.5)
11
(±1.0)
13
(±1.0)
15
(±1.0)
17
(±1.0)
19
(±1.0)
21
(±1.0)
23
(±1.0)
25
(±1.0)
27
(±1.0)
Year of
OC
deposition (±0.01)
14
C
14
C
age
δ13C
Iodine
(‰)
-21.96
(±0.004)
-21.81
(±0.1)*
-21.67
(±0.01)
-
(ppm)
-
(%)
-
(‰)
-
(years)
-
1.20
1.07
-177
(±5)
-
1560
(±50)
-
1987
1.11
-
-
1986
-
-
-
1985
1.31
-
-
1984
-
-
-
1.22
-
-
-
-
-
1.34
-
-
1977
1.41
-
-
1975
1.41
-
-
1973
1.40*
-
-
-
-
-
1964
0.95
-
-
1963
-
1962
-
-245
(±7)
-
2260
(±70)
-
1961
0.94
-
-
-
-
-
1993
1980
11
-21.70
(±0.01)
-21.72
(±0.01)
-21.76
(±0.02)
-
11.5
(±0.5)
11.6
(±0.5)
14.5
(±1.0)
-
-21.94
(±0.01)
-
-
-21.65
(±0.02)
-
11
(±1.0)
-
-21.54
(±0.02)
-21.52
(±0.03)*
-21.49
(±0.02)
-
5.7
(±0.3)
-
-21.55
(±0.07)
-
7.1
(±1.2)
-
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29
(±1.0)
31
(±1.0)
33
(±1.0)
35
(±1.0)
37
(±1.0)
39
(±1.0)
41
(±1.0)
43
(±1.0)
45
(±1.0)
47
(±1.0)
49
(±1.0)
51
(±1.0)
53
(±1.0)
55
(±1.0)
57
(±1.0)
59
(±1.0)
61
(±1.0)
63
(±1.0)
65
(±1.0)
67
(±1.0)
69
(±1.0)
71
(±1.0)
73
(±1.0)
1960
1959
1957
-
-
-
-
0.92*
-
-
-
0.89
-
-
0.92
-
-
-21.47
(±0.02)
-
0.95*
-
-
0.97
-
-
0.98*
-
-
8.9
(±1.3)
5.3
(±0.3)
3.4
(±0.3)
26.9
(±0.5)
34.3
(±1.8)
6.8
(±1.2)
15.5
(±1.8)
9.2
(±1.6)
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
1.15
-
-
-
-
-
-
-
-
1.20
-
-
-21.35
(±0.02)
-
-
-
-
0.95
-334
(±7)
-
-
-
-
-
-
0.97*
-
-
-21.40
(±0.04)
-
-
-
-
-
0.99*
1.00
1.07
1955
1953
-21.38
(±0.01)
-
-21.28
(±0.02)
-364 3640
-21.23
(±10) (±130) (±0.04)*
-21.19
(±0.02)
-
-21.46
(±0.02)
3260
-21.46
(±130) (±0.02)*
-21.46
(±0.02)
-
12
10.8
(±0.2)
8.9
(±0.4)
24.8
(±2.1)
-
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75
(±1.0)
77
(±1.0)
79
(±1.0)
81
(±1.0)
83
(±1.0)
85
(±1.0)
87
(±1.0)
-
-
-
0.98*
-
-
0.99
-
-
-
-
-
-
-
-
-
-
-
0.99*
-350 3460
(±10) (±130)
13
-21.50
(±0.02)
-21.87
(±0.02)
-
9
(±1.0)
-
-22.00
(±0.03)
-
-
-21.96
(±0.01)
22.8
(±1.3)
-
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Figure captions
Figure 1. Sampling site on Mississippi River used for seasonal survey and yearly reoccupation.
Core location in Gulf of Mexico: core 93P13 #2 = 28°55.5’ N, 89° 40.6’ W in about 60 m water
depth (Oktay et al., 2000), with new carbon isotopic results described here.
Figure 2. Downcore variations of
14
C (a) and δ13C (b).
Figure 3. Percent organic carbon (OC) and total
depth.
127
Iodine concentration (ppm) as a function of
Figure 4. Total iodine concentrations as a function of % OC, demonstrating a trend line of
increasing iodine with increasing organic carbon concentrations, with the outliers consisting of
data points from the layers that were deposited episodically and that were inferred to contain
peat-OC.
Figure 5. Linear relationship between
1950-1993.
14
C age (years) and δ13C (‰) for data points between
14
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Fig. 1
15
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Fig. 2
16
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Fig. 3
17
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Fig. 4
18
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Fig. 5
19